Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface, showing the new design.

The broken lines around the FIGURE show a display screen or portion thereof that forms no part of the claimed design. The broken lines within the FIGURE show portions of the graphical user interface that form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 